It is an object of the present invention to provide a resonator comprising a parallelepipedal bar of a piezoelectric material adapted to vibrate in an extension mode and suspension means for said bar forming a single piece integral with said bar and adapted to connect said bar to a base.
Resonators of this type are in particular described in patent GB-B-1,560,537 and U.S. Pat. No. 4,608,510 and will not be described again in detail here.
It will simply be noted that all these known resonators have bar suspension means comprising two arms perpendicular to the longitudinal axis of the bar that are symmetrical to one another in relation to this axis, each having one extremity connected to one of the lateral faces of this bar, substantially at the mid-point of its length.
These suspension means moreover have supports parallel to the longitudinal axis of the bar, also symmetrical in relation to this axis, and connected to the other extremity of the above- mentioned arms. Together with additional parts, these supports form a frame completely surrounding the bar, as in FIG. 1 of patent GB-B-1,560,537 already mentioned, or only half thereof, as in FIG. 5 of the same patent or in FIG. 4 of U.S. Pat. No. 4,608,510 also already mentioned.
The electrical connection terminals of the resonator, which also serve to fix it mechanically to a base, are disposed on this frame and conductive tracks connecting these terminals to the vibration excitation electrode of the bar are also disposed, in part, on this frame and extend onto the above mentioned bars.
A resonator of this type generally displays good characteristics, notably a high quality factor. What is more, it is also relatively easy to mass produce, notably using conventional processes involving photolithographic and chemical etching techniques, especially when it resembles the resonator described in U.S. Pat. No. 4,608,510. It consequently has a relatively low cost price.
Nevertheless, a resonator of this type, in common with all resonators regardless of their type, is a very complex vibrating system.
Each of its components, and in particular each of the elements of the suspension means of its bar, can in fact, be caused to vibrate in a large number of different modes which, with the exception of the main extension mode in which the vibration of the bar is excited, are all parasitic modes.
These parasitic modes have frequencies and amplitudes which it is virtually impossible to predict in advance because they depend on the precise dimensions of various components of the resonator, notably on its bar suspension means. However, there are always small differences in these dimensions from one resonator to another, even within one production batch. It has also been found that these frequencies and these amplitudes depend quite considerably on the way in which the resonator is fixed to the base and on the amplitude of the excitation signal applied thereto by the oscillator circuit to which it is connected. Moreover, these frequencies and these amplitudes vary as a function of the temperature in a different manner from one mode to another.
When the frequency of one of these parasitic modes is close to the frequency of the main vibration mode of the bar or of one of its harmonics, this parasitic mode can be coupled to this main mode and, if its amplitude is sufficiently large, it can absorb a not negligible part of the energy of the main mode. Coupling of this kind can modify the frequency of vibration of the resonator and/or impair its characteristics, notably its quality factor, it even being possible for this deterioration to lead to cessation of the oscillator circuit of which this resonator forms part.
The risk of coupling of this type is particularly great in the known resonators described above. In fact, the variations in the length of the bar whilst this vibrates in its main vibration mode are inevitably accompanied by variations in its width. In other words, the bar vibrates simultaneously in its main extension mode and in a secondary transverse mode, these two modes being perfectly coupled and having exactly the same frequency.
Since the bar suspension means are solidly connected to the lateral faces thereof which are displaced by this transverse vibration, this latter can clearly be very easily coupled with one or other of the parasitic vibrations which can originate in these suspension means, notably in the supports parallel to the length of the bar.
It emerges from the foregoing that all these known resonators have characteristics, and variations thereof as a function of temperature, which are not entirely foreseeable. All these resonators therefore have to be tested after manufacture throughout the entire temperature range within which they are to operate because, even within one and the same production batch, some of them may display, at certain temperatures, a variation in their frequency of vibration and/or a reduction in their quality factor in excess of permissible tolerances. These latter resonators naturally have to be scrapped. The cost of these tests and the reduction in output due to this scrapping of defective resonators naturally increases the cost price of these resonators.
Further, before commencing mass production of a new resonator, even if this differs only slightly from an already produced resonator, numerous tests have to be carried out on this new resonator in order to check its characteristics. These tests frequently show that these characteristics are not exactly what is required. It is then necessary to change the shape and/or the dimensions of one or other of the components of this new resonator and then to repeat the tests to determine whether these modifications have had the desired effect. This process sometimes has to be repeated several times.
These tests and modifications are, of course, lengthy and costly and further increase the cost price of these known resonators.
U.S. Pat. No. 3,906,260 describes a resonator comprising a bar designed to vibrate in torsion about its longitudinal axis and a single suspension arm from this bar which forms a single, integral part with this latter. One of the extremities of this arm is connected to one of the faces of the bar, at the middle of the length of this bar, and the other extremity of this arm, which carries the electrical contact terminals of the resonator, is fixed by means of welding directly onto connection wires which cross a base forming part of the resonator housing.
Experience has shown that a resonator comprising a bar designed to vibrate in extension and a single bar resembling that of the resonator described in this patent US-A-3,906,260 presents substantially the same disadvantages as the previously mentioned resonators. This is due to the fact that the welding of the contact terminals of the resonator on the wires crossing the base cannot present precisely the same characteristics from one resonator to another and that this base itself can be the seat of parasitic vibrations having frequencies which cannot be predicted any more than can their variation as a function of temperature. What is more, the frequencies of these parasitic vibrations depend on the way in which the encapsulated resonator is mounted in its ultimate location.